Joseph A. Fournier, Robert K. Bohn,John A. Montgomery, Jr.

University of Connecticut, Storrs, CT

Microwave Spectroscopy and Structures of Perfluorohexane and

1H-heptafluoropropane

M. D’Amore, et. al., J. Am. Chem. Soc. 128(2006)1099.

C. Sperati, H. Starkweather, Jr. Fortcher. Hochpolym. Forsch. 2(1961)465

} d

ρ

θ

τ2345φr

T. Shimanouchi, S. Mizushima. J. Chem. Phys. 23(1955)707.

M. Iwasaki. J. Polym. Sci. A-1 (1963)1099.

Helical Perspective Molecular Perspective

Helical vs. Dihedral AngleHelical Perspective (X-ray crystallographers) Molecular Perspective

ρ – distance from each atom to the helix axis r – CC bond length

θ – angle of rotation about the helix axis φ – CCC bond angle

d – pitch; translation along the helix axis τ – CCCC dihedral angle

Helix parameters as functions of molecular parameters:cosθ = ½[ - cosφ + cosτ - cosφ cosτ - 1]

d2 = r2(1 - cosτ)(1 - cosφ)/(3 + cosφ - cosτ + cosφ cosτ)

ρ2 = 2r2(1 + cosφ)/(3 + cosφ - cosτ + cosφ cosτ)2

Molecular parameters as functions of helical parameters:r2 = d2 + 4ρ2sin2(θ/2)

cos(φ/2) = (1 - d2/r2)1/2 sin(θ/2)

tan(τ/2) = (d/r) tan(θ/2)

b

a

c

Top view, Helical Twist, C2 symmetry

Perfluoropentane, C5F12

J.A. Fournier, R.K. Bohn, J.A. Montgomery, Jr., M. Onda. J. Phys. Chem. A 114 (2010) 1118.

Pentane (C5H12), all bonds trans (180O), C2v symmetry

C5F12, bonds twisted from trans by 17O, C2 symmetry

Perfluorohexane, C6F14

Hexane (C6H14), C2h C6F14, C2, ~17o twist

a

b

c

Observed Spectroscopic Constants

 A/MHz 824.9001(9)B/MHz 202.2195(8)C/MHz 198.3355(10)DJ/kHz 0.0691(4)Paa/uÅ2 2217.304Pbb/uÅ2 330.798Pcc/uÅ2 281.857Kappa -0.9876No. Lines 46Std. Dev./kHz 1.8

Scaling the Calculated Model• Scale the principal coordinates of each atom from the PBE0/VTZ calculations by

the factor (Pii obs / Pii PBE0)1/2

• Scaling Factors: 0.99996(a), 1.00167(b), 1.00288(c).

• Calculate structural parameters from new coordinates.

  Obs'd PBE0/VTZScaled

PBE0/VTZ  

A/MHz 824.900 828.6 824.90

B/MHz 202.220 202.3 202.22

C/MHz 198.336 198.4 198.34

Paa/uÅ2 2217.304 2217.5 2217.30

Pbb/uÅ2 330.798 329.7 330.80

Pcc/uÅ2 281.857 280.2 281.86

Scaled Structure

  PBE0/VTZScaled

PBE0/VTZC1-C2 1.555 1.556C2-C3 1.558 1.559C3-C4 1.561 1.561/ C1C2C3 114.0 113.9/ C3C4C5 113.0 112.9C1C2C3C4 16.7 16.7C2C3C4C5 18.0 18.0

1H-heptafluoropropane

HCCC Trans HCCC Gauche

J.A. Fournier, R.K. Bohn. Dalton Trans. 39(2010) 4575.

Perfluoropropane, C3F8

PBE0/VTZ computations predict a non-helical, C2v structure. The computed spectroscopic constants are in excellent agreement with the observed.

J.A. Fournier, R.K. Bohn, J.A. Montgomery, Jr., M. Onda. J. Phys. Chem. A 114 (2010) 1118.

Observed Spectroscopic Constants

  Gauche TransA/MHz 1995.4656(7) 1752.4998(10)B/MHz 1120.2799(4) 1184.3437(19)C/MHz 982.7300(5) 1137.0414(20)ΔJ/kHz 0.0691(4) 0.0938(20)ΔJK/kHz 0.0315(11) 0.763(10)ΔK/kHz 0.0378(14) -0.758(8)δJ/kHz 0.01578(19) 0.0092(18)δK/kHz -0.238(3) -1.12(8)Kappa -0.728360 -0.846284No. Lines 71 38a-types 8 12b-types 18 26c-types 45 0Std. Dev./kHz 1.1 1.7

Dipole Moments

Compound Predicted Dipole/D DihedralC4F10 0.03 14o

C5F12 0.08 17o

C6F14 0.06 17-18o

0.8 17o

0.7 14o

Trans 1H-HFP

0.4 7o

0.2 3o

0.06 1o

Scaling the Calculated Model• Scale the principal coordinates of each atom from the PBE0/VTZ calculations by

the factor (Pii obs / Pii PBE0)1/2

• Scaling Factors: 1.00033(a), 1.00268(b), 1.00122(c) for gauche form.

• Calculate structural parameters from new coordinates.

  Obs'd PBE0/VTZScaled

PBE0/VTZ   Obs'd PBE0/VTZScaled

PBE0/VTZ

A/MHz 1995.4656 2004.0 1995.46 1752.4998 1760.8 1752.50

B/MHz 1120.2799 1121.4 1120.28 1184.3437 1184.5 1184.34

C/MHz 982.7300 984.8 982.72 1137.0414 1138.6 1137.04

Paa/uÅ2 356.060 355.8 356.06 291.405 291.8 291.40

Pbb/uÅ2 158.205 157.4 158.21 153.064 152.1 153.06

Pcc/uÅ2 95.059 94.8 95.06 135.312 134.9 135.31

Gauche Trans

Scaled Structures

Gauche

Trans  PBE0/VTZ Scaled   PBE0/VTZ ScaledC1-C2 1.536 1.538 1.535 1.535C2-C3 1.545 1.546 1.546 1.548/ C1C2C3 114.0 113.9 117.0 116.9HC1C2C3 60.8 60.9 180 180F4C1C2C3 179.1 179.1 60.1 60.1F9C3C2C1 170.9 170.9 180 180

Summary• The microwave spectra of the C2 structure of

perfluorohexane and the gauche and trans conformers of 1H-heptafluoropropane have been observed and assigned.

• The dihedral angles of C6F14 are ~17-18o away from trans.

• No c-type transitions were observed for the trans form, indicating an ab-plane of symmetry or an insufficient twist.

• Structural analysis was performed by scaling the calculated models to reproduce the observed spectroscopic constants.

Acknowledgments

• At UConn: Prof. Harvey Michels, James Dombrowski, Congtin Phan.

• At Wesleyan : Prof. Stewart Novick, Prof. Pete Pringle, Dr. Andrea Minei, Dan Frohman.

• Measurements were performed on the pulse-jet Fourier Transform Microwave Spectrometer of the Southern New England Microwave Consortium at Wesleyan University.